1. Field of the Invention
The present invention relates generally to construction of sheeting having the capability of retroreflecting incident light and, more particularly, to a cellular retroreflective sheeting having enhanced reflective characteristics through wide angles of incident light while, at the same time, having improved bonding strength of its composite layers.
2. Description of the Prior Art
Retroreflective devices have been advantageously used for many years to improve highway safety in many parts of the world. Devices such as pavement markers, automobile reflectors, post markers and highway signage, for example, have been constructed using various retroreflective devices which reflect incident light from vehicle headlight beams back to the driver and serve during low ambient light driving conditions to inform the driver of approaching danger or other highway conditions.
One form of retroreflective device is known as a cube-corner type reflector. This device typically comprises an array of several "cube" elements each consisting of three mutually perpendicular faces which serve to receive incident light and retroreflect the light through 180.degree. approximately parallel to its incident path and back to its source. The term "cube-corner" has long been recognized in the art to refer to essentially any structure of three mutually perpendicular faces without regard to the size or shape of each face or the optical axis of the element so provided. An early example of a cube-corner type reflector is disclosed in Stimson, U.S. Pat. No. 1,906,655, issued May 2, 1933. Another example is in Hennan, U.S. Pat. No. 3,332,327 issued Jul. 25, 1967, both of which teach a pavement marker construction.
In pavement marker construction, the cube corner elements may be relatively large in size because the marker is constructed to be a rigid assembly capable of withstanding vehicle loads and tire impact. However, an important application for cube-corner type reflective elements has been developed in the area of retroreflective sheeting. Retroreflective sheeting is particularly useful in the construction of highway signage, for example, in which an aluminum sign blank is covered with a layer of light reflective sheeting bearing suitable indicia for informing drivers of a particular highway condition.
Unlike pavement marker applications for cube-corner type reflective elements, in reflective sheeting applications the cube-corner elements are reduced in size to be useable on a relatively thin film substrate. Preferably, the sheeting must be flexible and capable of being produced and supplied in roll form. To this end, methods have been developed to form thin film materials with retroreflective elements such as by embossing or casting. Typically, the sheeting comprises a transparent acrylic substrate, or film, although various other forms of thermoplastic material may be used as polycarbonate, vinyl, polyethylene or polyurethane, for example.
An example of a highly efficient method and apparatus for continuous embossing of a resinous film with cube-corner retroreflective elements is disclosed in Pricone et al., U.S. Pat. No. 4,601,861, the disclosure of which is incorporated herein by reference. In this process, a continuous web of transparent film is fed through an embossing machine in which the film is heated to a transition temperature and compressed by an embossing tool such that resinous film material flows into the pattern of the tool. The film is then cooled, quenched and stripped from the tool. The tool may be constructed by a process of the type disclosed in Montalbano, U.S. Pat. No. 4,460,449, the disclosure of which is incorporated herein by reference. The tool of this patent is capable of creating very small, accurately formed cube-corner elements on the order of several thousand per square inch of film.
It is well known that cube-corner elements are orientation sensitive and have varying degrees of reflectivity depending upon the angle of the incident light. Accordingly, one generally accepted practice in the construction of cube-corner sheeting is to construct master tool blocks on the order of one-quarter inch or so square with the cube prisms tilted on the order of approximately six degrees. Then, using multiple master blocks, each in a different rotational orientation, multiple matrices or grids of cube-corner reflective elements may be formed in the film such that the resulting sheeting is highly reflective, overall, over a wide range of orientation angles.
In using a cube-corner system to construct retroreflective sheeting, it is generally understood that the rear surfaces of the cube elements must either be metalized or supported with an air gap between any adjacent rear support surface. This is so because essentially any material other than a reflective metal coating placed in intimate contact with the rear surfaces of the cubes will have a refractive index such that the focal point of the cube element will be altered. Accordingly, metalization of the rear surfaces of cube-corner elements has been practiced to allow the element substrate to be bonded to a suitable rear support layer. In practice, a rear support layer is essential to protect the reflective elements and provide a sheeting surface which may be readily secured as with adhesive to a sign blank or the like. Metalization may readily be performed by known aluminum or silver vapor deposition techniques, for example.
A known disadvantage of metalization in the construction of retroreflective sheeting is that the metallic reflective layer imparts a distinct grayish tint to the resulting sheeting. This grey appearance of the sheeting has been found undesirable for highway signage, particularly during daytime light conditions. An air gap system, in which the cube elements are spaced from the rear layer, on the other hand, creates a sheeting having enhanced brilliance over metalized products. Accordingly, attempts have been made to construct sheeting using an air gap system in some form.
In one form of an air gap system as disclosed in Pricone, U.S. Pat. No. 4,618,518, a web of transparent thermoplastic material is first embossed with cube-corner reflective elements and the elements are overcovered in a pattern as by printing with a slurry of mineral spirits, alcohol and hydrophobic silica powder to create small islands of silica covering the surfaces of a number of the cube corner elements. Because the silica is water-resistant, a layer of water based acrylic is then flooded over the silica creating a continuous surface coating and forming cell walls around the silica in a desired printed pattern. The silica granules, which contact the cube faces only tangentially, maintain an air gap and allow the elements to be hermetically sealed by the acrylic coating. Thus, an acceptably reflective sheeting product may be achieved in which the sheeting has a suitable rear support layer, while employing the desired air gap system. An appropriate cell size is chosen so that breaking the seal of the cells by cutting the sheeting to size will result in only minor contamination of reflective elements at the very edges of the sheeting form and an edge sealing step is avoided. However, a disadvantage of this construction is that to obtain adequate mechanical bonding of the acrylic support layer, the cell walls formed around the silica filler, in practice, must have a wall thickness on the order of 0.035 inch (0.9 mm). This is necessitated by the extreme conditions under which the resulting sheeting is expected to perform, such as rapid and severe expansion and contraction of an associated sign blank, for example. Moreover, with cell walls of at least 0.035 inch (0.9 mm) thickness, it has been found that the resulting sheeting with silica backing has reduced efficiency at 30.degree. and greater incident angles. Accordingly, this type of sheeting may not meet certain reflectivity standards for use in some highway signage applications.
In another form of an air gap system as used with cube-corner reflective elements, a web of reflective elements is formed having cell walls surrounding a plurality of cube elements. A flat cover sheet is applied covering the cell walls and the assembly is chemically and/or heat fused to bond the web to the cover sheet. The result is a sheeting having many small cells defining air gaps between the reflective elements and the cover sheet. An example of this construction is disclosed in McGrath U.S. Pat. No. 4,025,159. However, a disadvantage of this construction is that when heat treating is used to bond the web and cover layer the process is very difficult to control to avoid heat from distorting the cube-corner elements adjacent the cell walls during bonding. Cube-corner elements inherently must have their three reflective faces oriented within a few minutes of perpendicular or else they cannot be retroreflective. Thus, the heat needed to bond the reflective web and cover layer, which is necessarily in the range of 200-500.degree. F., can destroy enough cube-corner elements as to render 20-30% of the resulting sheeting product completely non-reflective. Moreover, when chemical processes are used to bond the web to the cover sheet, not only are these processes complicated and time consuming they typically are accompanied by adverse environmental impact if not carefully controlled. Another version purportedly of flexible sheeting is disclosed in U.K. Patent No. 1,476,447, also referencing heat sealing.
In a more recent patent application filed under the Patent Cooperation Treaty as Serial No. WO95/11464, published Apr. 27, 1995, some passing reference is made to using radio frequency or ultrasonic welding to form cells of cube-corner retroreflective elements on retroreflective sheeting. However, this reference is merely prophetic and no associated disclosure of such welding processes is provided. That application in fact describes the best mode as being thermal fusion techniques. Indeed, in practice, it has not been heretofore known to use radio frequency or ultrasonic welding techniques to continuously manufacture commercially viable retroreflective sheeting products.
Accordingly, it is desirable to provide retroreflective cube-corner type sheeting using an air gap system. It is further desirable to provide such sheeting in which arrays of small cells each enclose plural reflective elements of a substrate film in a hermetically sealed construction wherein only minor contamination of the sheeting edges can occur after the sheeting is cut to size.
Still further, it is desirable to provide such sheeting in which a cover layer is firmly bonded to the substrate layer such that the cover layer will not detach from the associated substrate layer under adverse conditions encountered, for example, in highway signage applications.
It is further desirable to provide such sheeting in which the bonding of the substrate and cover layers will not affect the retroreflectivity of the cube corners as the incident angle of the light is increased. Further, it is desirable to provide such sheeting in which the substrate and cover layers are bonded without appreciable destruction of cube-corner reflective elements whereby the number of functioning elements in the sheeting is correspondingly maximized.
Further, it is desirable to provide such sheeting which is capable of being produced continuously at high production rates and which does not involve potential adverse environmental effects in the production process.